The present invention relates to a solid electrolytic capacitor made by a sintered body of valve action metal powders such as tantalum powders and a method of fabricating the same. More particularly, the invention relates to a solid electrolytic capacitor having a structure such that a cathode layer of a capacitor element is not peeled off, equivalent series resistance (impedance) is not increased, and short circuit or the like do not occur due to a temperature shock at the time of solder reflow when the capacitor is mounted on a circuit board or the like and to a method of fabricating the same.
A capacitor element 1 to be built in a solid electrolytic capacitor such as a conventional tantalum electrolytic capacitor is fabricated as follows. As shown in FIG. 3, by using, for example, Ta powders, a molded body of a rectangular parallelepiped or a cube shape in which an anode lead 11 such as a Ta wire is embedded in one of the wall faces is obtained. The molded body is sintered in a vacuum atmosphere. Around the powders and the sintered body 10, a dielectric layer (oxide film) 14 made of Ta2O5 or the like and a manganese dioxide layer 15 are formed. Further, a graphite layer 16, and a silver resin layer 18 are sequentially formed, thereby obtaining an enveloping layer 19. The surface of the enveloping layer 19 is used as a cathode 12. The dielectric layer 14 is formed by anodizing (conversion treatment (coating)) the sintered body 10. The manganese dioxide layer 15 is formed by soaking the sintered body 10 in a manganese nitrate solution and thermal decomposing the manganese nitrate solution. The graphite layer 16 is formed by applying aqueous graphite (what is called India ink) and sintering it at about 130xc2x0 C. The silver resin layer 18 is formed by applying what is called a silver paste and drying it at about 200xc2x0 C. Reference numeral 13 denotes a Teflon ring.
As described above, the cathode 12 of the capacitor element 1 is formed by providing the enveloping layer 19 consisting of the above layers around the sintered body 10. The coefficients of linear expansion of the layers are different from each other as shown in Table 1.
As described above, in the cathode of the capacitor element, the various layers are provided on the surface of the sintered body on which the dielectric layer is formed and their coefficients of linear expansion are different from each other. Consequently, by a stress of a temperature shock of solder reflow at the time of mounting the solid electrolytic capacitor on a circuit board or the like, each of the layers is peeled or floated. As a result, the impedance (equivalent series resistance at a high frequency) increases and a problem such as deterioration in electric characteristics arises.
The invention has been achieved to solve such problems and an object of the invention is to provide a solid electrolytic capacitor having a small impedance between the anode and the cathode by preventing an increase in impedance (equivalent series resistance) based on a thermal stress which occurs between layers in the cathode layer, and a method of fabricating the solid electrolytic capacitor.
The inventor of the present invention has eagerly examined the cause of an increase in impedance between the anode and the cathode or an increase in leakage current after a solid electrolytic capacitor is mounted on a circuit board or the like. As a result, the inventor has found the cause as follows. At the time of mounting, the solid electrolytic capacitor is put in a reflow oven for soldering and the temperature is increased to about 270xc2x0 C. By the temperature shock at the time of increasing the temperature, peeling or light floating occurs in each of the interfaces of the manganese dioxide layer, the graphite layer, and the metal layer (silver resin layer) as the cathode layers, the series resistance between layers therefore increases, and the impedance rises. It was found that a failure often occurs in especially the graphite layer which is a very thin layer, and the failure directly exerts a very large influence on the impedance between the anode and the cathode.
Further, the inventor examined the causes of the occurrence of peeling and floating between layers at the time of reflow for soldering and found main two causes which are: different coefficients of linear expansion of the layers; and a reversal of the relation between a stress on each layer in a state where the temperature is returned to a room temperature after sintering for forming the layers and a stress between layers at the time of soldering reflow. By interposing a resin graphite layer having an intermediate coefficient of linear expansion especially between the graphite layer and the metal layer (which is formed by drying a material such as a silver paste, obtained by mixing metal powders in a resin and which contains a small amount of a resin, so that it can be also called, for example, a silver resin layer) of different coefficients of linear expansion, and/or by sintering the graphite layer at a temperature of about 230 to 300xc2x0 C., which is the temperature of the reflow for soldering, a burden on the graphite layer which is easily influenced by the stress is lightened. Consequently, an increase in impedance can be prevented.
According to the invention, there is provided a solid electrolytic capacitor having a capacitor element which comprises: a sintered body of valve action metal powders, in which an anode lead is embedded in one of wall faces; a dielectric layer provided on the metal powders; a manganese dioxide layer formed on the outer surface of the sintered body which is provided with the dielectric layer on the metal powders; a graphite layer formed on the manganese dioxide layer; and a metal layer formed on the graphite layer, wherein the graphite layer is made by an aqueous graphite layer provided on the manganese dioxide layer side and a resin graphite layer provided on the metal layer side.
The aqueous graphite denotes conventionally used graphite and is obtained by dispersing carbon powders in the water. It denotes what is called India ink. The resin graphite is obtained by dispersing carbon powders in a resin such as phenol resin and denotes what is called a paint.
In this structure, as the metal layer is formed by applying a paste obtained by mixing metal powders and a resin such as a silver paste and drying the paste, the metal layer therefore contains a small amount of resin. The resin graphite has a coefficient of linear expansion which is close to that of the metal layer. Between the aqueous graphite layer and the metal layer having coefficient of linear expansion which are particularly different from each other, the resin graphite layer having an intermediate coefficient of linear expansion is provided. As a result, the action of a stress based on the difference between the coefficients of linear expansion of the graphite layer and the metal layer is reduced. Even against a thermal shock which occurs during soldering at the time of mounting the capacitor, the layers of the cathode are not peeled or the like.
The aqueous graphite layer is formed by, for example, applying a solution obtained by dispersing carbon powders in water and burning the solution. The resin graphite layer is formed by, for example, applying a resin mixed with carbon powders and burning the resin.
Each of the aqueous graphite layer and the resin graphite layer is burned, preferably, at a temperature ranging from 230 to 300xc2x0 C. In this case, the peeling between the graphite layer and neighboring layers does not easily occur even against a temperature cycle of the soldering reflow at the time of mounting the capacitor.
The metal layer is formed by applying a resin mixed with metal powders and drying the resin.
In the case of the structure such that each of the anode lead of the capacitor element and the metal layer is electrically connected to an end of an external lead, the whole capacitor element is molded in a package made of a resin, and the external lead is formed so that the other end of the external lead is soldered, the invention is especially preferable since the capacitor is rigid at the time of soldering.
The method of fabricating a solid electrolytic capacitor according to the invention, comprising the steps of: (a) forming a sintered body of valve action metal powders, (b) forming an oxide film on the surface of the metal powders by anodizing the sintered body, (c) forming a manganese dioxide layer around the metal powders and the sintered body, (d) forming a graphite layer on the manganese dioxide layer around the sintered body, and (e) forming a metal layer on the graphite layer, wherein the graphite layer is formed by applying a liquid graphite and burning the liquid graphite at 230 to 300xc2x0 C.
By setting the temperature at which the graphite is burned to about the temperature at the time of soldering reflow, in a state where the temperature decreases to a room temperature after burning, the compressing force acts on the material having a small coefficient of linear expansion and the tensile force acts on the material having a large coefficient of linear expansion. At the time of soldering reflow, however, since the temperature is almost the same as that at the time of burning, no stress acts. That is, different from the conventional technique, the relation of stresses on layers at the room temperature and that at the time of reflow are not inverted. A damage is therefore reduced.
Preferably, the graphite layer is formed by applying an aqueous graphite, burning the aqueous graphite at 230 to 300xc2x0 C., applying a resin graphite on the aqueous graphite, and burning the resin graphite at 230 to 300xc2x0 C. By the method, floating which occurs in the cathode layers can be prevented more effectively.